“Carbonate ester” is a generic name of a compound obtained as a result of one atom or two atoms among two hydrogen atoms of carbonic acid, CO(OH)2, being substituted with an alkyl group or an aryl group, and has a structure of RO—C(═O)—OR (R and R′ each represent a saturated hydrocarbon group or an unsaturated hydrocarbon group).
A carbonate ester is used as an additive, for example, a gasoline additive for improving the octane value and a diesel fuel additive for decreasing the amount of particles in exhaust gas. A carbonate ester is also used as, for example, an alkylation agent, a carbonylation agent, a solvent or the like usable for synthesizing resins or organic compounds such as polycarbonate, urethane, pharmaceutical drugs, agricultural chemicals or the like, a material of an electrolytic solution of lithium ion cells, a material of lubricant oil, or a material of an oxygen absorber for rust inhibition of boiler pipes. As can be seen, a carbonate ester is a very useful compound.
According to a conventionally mainstream method for producing a carbonate ester, phosgene, which is used as a source of a carbonyl, is directly reacted with an alcohol. Phosgene used in this method is highly hazardous and highly corrosive, and therefore, needs extreme caution to be handled, for example, transported or stored. It is highly costly to control and manage, and guarantee the safety of, production facilities of phosgene. According to this method, the materials and catalysts used for producing a carbonate ester contain halogen such as chlorine or the like, and the produced carbonate ester contains a trace amount of halogen, which is not removed by a simple purification step. When the carbonate ester is used for a gasoline additive, a light oil additive or an electronic material, such halogen may undesirably cause corrosion. Therefore, a thorough purification step is indispensable to decrease the trace amount of halogen in the carbonate ester to the level of an extremely trace amount. In addition, recently, administrative offices provide a strict administration guidance and do not permit new establishment of production facilities using this method because phosgene is highly hazardous to the human body. In such a situation, a new production method of a carbonate ester that does not use phosgene is strongly desired.
There is another known method for producing a carbonate ester. According to this method, a carbonate ester is directly synthesized from an alcohol and carbon dioxide using a heterogeneous catalyst. Regarding this method, studies had been on using 2-cyanopyridine or benzonitrile as a wettable powder to significantly improve the production amount and the production speed of the carbonate ester, to allow the reaction to advance easily at a pressure close to normal pressure, and to increase the reaction speed (see Patent Documents 1 and 2). However, there was a problem regarding the method for treating or using benzamide or the like generated as a by-product.
For example, the use of benzamide generated by the reaction of benzonitrile and water is limited to being for some of pharmaceutical and agrochemical intermediates. Therefore, regarding the method of producing a carbonate ester using benzonitrile as a wettable powder, benzamide generated as a by-product is desired to be regenerated into benzonitrile and reused. It is now an issue to realize a regeneration reaction with a high level of selectivity (because it is considered that if a by-product is generated, benzonitrile is not easily used as a wettable powder) and a high yield (because if the yield is low, benzamide remains in a large amount, which increases the amount of work, namely, work load, of separating benzamide and benzonitrile from each other).
In light of the above-described situation where regeneration of benzamide or the like into benzonitrile or the like involves problems, there is a known method for realizing the regeneration with no use of a strong reagent and with the generation of a by-product being suppressed (Patent Document 3).
However, according to this method, generation of nitrile by dehydration of an amide compound requires 400 hours and thus is not well balanced to be adopted with a carbonate ester synthesis reaction, which requires only 24 hours. This method also has a problem that steps of extraction, infiltration and the like are necessary for solid-liquid separation of a catalyst, which increases the number of production steps and complicates the production process.